For anyone experimenting with mind-altering drugs, the prospect of having a “bad trip” is a rather unsettling one. But what if the trip never ends? What happens when drugs are taken and the drugs have metabolized fully from your system, but the effects don’t dissipate? What would it be like to never stop tripping?

For sufferers of Hallucinogen persisting perception disorder (HPPD), that’s a question they don’t have to ask themselves, because they live it every day.

The mechanism behind HPPD isn’t entirely clear, but what science does know is that it’s not the same thing as “acid flashbacks” — once someone has taken LSD (or other mind-bending hallucinogens, like peyote) they can occasionally have psychological flashbacks (similar to post-traumatic stress disorder) to some of the more unsettling aspects of their experience “tripping”. HPPD, on the other hand, consists of visual disturbances which don’t come and go. They’re constant and, unlike flashbacks, aren’t psychological. The sufferer knows that what they’re seeing isn’t real, and the disturbances are more like a short-circuit in perception rather than a bad memory.

These visual changes may have started during a trip, but once the drug has left the body, they persist and become part of the person’s waking life. Symptoms like “trailing” of objects, changes in color perception and motion-based experiences like “the walls are moving” are all common complaints of those with HPPD. Another common occurrence is the presence of “after images”, which happens when you look at an object, look away and still see it in your field of vision, usually in the negative colors scheme.

It’s actually not uncommon for all of us, drugs or not, to experience these kinds of visual disturbances when we’re overtired, ill, in poorly-lit spaces or other situations that stimulate our visual cortex. The difference is, for most of us these experiences are temporary and exceptionally fleeting; for those with HPPD it becomes how they perceive the world all the time, regardless of environment.

But why? That’s the question researchers have. It’s clear that there is a link between taking a psychedelic drug and developing HPPD, but it’s not as though everyone who has ever dropped acid develops HPPD. Some who have done drugs consistently for decades don’t develop any of these visual disturbances on a chronic basis and others, who have done the drugs but once, almost immediately develop symptoms consistent with HPPD.

What research has shown is that for those who do develop it, it tends to occur early on in their experimentation with psychedelic drugs. Since it doesn’t seem to be proportional to the amount of drugs taken, or the amount of time the drugs have been consistently used, it implies that some people may be predisposed to developing HPPD if and when they should experiment with mind-altering substances. What complicates this theory is that there does exist a group of HPPD sufferers who are long-term drug users who have experimented with multiple psychedelic drugs, which makes it nearly impossible to conclude which of them ultimately led to the development of HPPD.

Abby Norman is a writer based in New England, currently writing a memoir for Nation Books. Her work has been featured on The Rumpus, The Independent, Cosmopolitan, Medium, Seventeen, Romper, Bustle, and Quartz.